U.S. Pat. No. 5,241,034 describes alkenyl-containing siloxane copolymers which are prepared by reacting an organic compound having two, three or four terminal aliphatic double bonds with organopolysiloxane having Si-bonded hydrogen atoms in the presence of a hydrosilylation catalyst. In the resulting copolymers the organopolysiloxane blocks are connected via hydrocarbon bridges.
In accordance with DE 41 23 423, branched alkenylsiloxane polymers are prepared which carry at least two alkenyl groups. The siloxanes used for the polyaddition reaction (final stage) have no more than two terminal SiH groups, whereas prior SiH-functional siloxanes used analogously, due to their branching, carried more than two SiH groups.
Alkenylsiloxanes are also discussed DE 196 466 42. In the latter, the alkenylsiloxanes acquire their branching by virtue of T units of the general structure RSiO.sub.3/2, incorporated by equilibration. The DE 195 484 74 discloses branched vinylsiloxanes which contain T units of the formula O.sub.1/2 R.sub.2 SiYRSiO. DE 195 221 44 describes branched SiH precursors having siloxane segments of not more than 6 silicon atoms, which are reacted with an excess of .alpha.,.omega.-diene containing no siloxane units.
In DE 196 290 53 are disclosed polymers containing unsaturated end groups on siloxane segments which are attached directly to the organic branching unit. The preparation process involves complex hydrolytic steps and the use of halosilanes. DE 196 270 22 describes polymers which contain no organic branching units but instead customary siloxy structures as RSiO.sub.3/2 and SiO.sub.4/2.